The present disclosure relates to methods and devices for laser material processing, such as laser cutting and/or laser welding, of a workpiece
Depending on the application and availability of installations, various laser beam sources are used in laser material processing operations. For example, for laser cutting operations, CO2 lasers having a laser wavelength of approximately 10 μm have been preferably used in the past, whereas solid-state lasers having a wavelength in the range of about 1 μm are becoming increasingly useful for laser fusion cutting operations due to an increase in cutting speed and improved energy efficiency.
In order to further increase the efficiency of the method used, attempts are often made to maximize the amount of energy coupled from the laser beam to the workpiece. For example, in WO 2010/016028 A1, azimuthally polarized radiation is used for the laser cutting operation. Due to the adaptation of the polarization, the absorption at the irradiation front is increased.
Compared to cutting operations performed with a laser having a wavelength of approximately 10 μm (e.g., a CO2 laser), however, the cutting operations that use a laser having a wavelength of 1 μm produce results that have a relatively high cutting edge roughness, corrugation and burr formation. As the workpiece (e.g., a metal sheet) becomes thicker, those effects increase. When laser cutting or laser welding is performed using a wavelength in the range of 1 μm with random polarization, an undulating irradiation front that includes local spatial disturbances, such as waves, can be formed in the workpiece. As a result of the formation of the undulating irradiation front, the laser radiation is incident on the workpiece at different angles. Since the Fresnel absorption of the laser radiation is dependent on the angle of incidence, the different angles of incidence at the local spatial disturbances of the undulating irradiation front lead to poor local Fresnel absorption behavior. The poor absorption, in turn, promotes an increase in the local disturbances rather than an attenuation of them. This effect can result in a relative poor cutting edge quality.
In DE 10 2007 024 700 A1, the laser radiation strikes the irradiation front of a workpiece at relatively small angles of incidence, for example, angles of incidence of less than 80° for the laser material processing of steel at a wavelength of 1 μm. In addition to achieving a maximum Fresnel absorption, a lower gradient of the Fresnel absorption is achieved so that process instabilities are prevented or at least reduced. The relatively small angles of incidence are adjusted by introducing artificial imaging errors that increase a divergence angle of the laser beam compared to the divergence angles brought about by imaging errors of standard optical systems.
As a result, the absorption characteristic is adapted to the irradiation front by reducing the local angles of incidence in the focused laser beam. Since absorption is increased, higher process speeds can be achieved. However, the reduction in angular dependency of the absorption, which accompanies the increase of the absorption, does not increase the processing quality.
Rather, the reduction in angular dependency and the reduced angle of incidence enhances the production of the local disturbances (e.g., waves) and consequently reduces the cutting quality. In addition, the foregoing references do not take into consideration the temperature distribution over the entire irradiation front and the resulting melt flows that extend azimuthally around the laser beam.